Galactic year

Unit of time

The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy. One galactic year is approximately 225 million Earth years. The Solar System is traveling at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph) within its trajectory around the Galactic Center, a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds; that speed corresponds to approximately 1/1300 of the speed of light. The galactic year provides a conveniently usable unit for depicting cosmic and geological time periods together. By contrast, a "billion year" scale does not allow for useful discrimination between geologic events, and a "million year" scale requires some rather large numbers.

The following list assumes that 1 galactic year is 225 million years. TimeEventGalacticyears(gal)Millionsof years(Ma)Past (years ago)About 61.32 gal Big BangAbout 54 gal Birth of the Milky Way20.44 gal Birth of the Sun17–18 gal 3937 MaOceans appear on Earth16.889 gal 3800 MaLife begins on Earth15.555 gal 3500 MaProkaryotes appear12 gal 2700 MaBacteria appear10 gal 2250 MaEukaryian period first appearance of eukaryotes Stable continents appear6.8 gal 1530 MaMulticellular organisms appear2.4 gal 540 MaCambrian explosion occurs2 gal 500 MaThe first brain structure appears in worms1.11 gal 250 MaPermian–Triassic extinction event0.2933 gal Cretaceous–Paleogene extinction event0.0013 gal Emergence of anatomically modern humansFuture (years from now)0.15 galMean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.1 galAll the continents on Earth may fuse into a supercontinent. Three potential arrangements of this configuration have been dubbed Amasia, Novopangaea, and Pangaea Ultima.2–3 galTidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible4 galCarbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out15 galSurface conditions on Earth are comparable to those on Venus today22 gal The Milky Way and Andromeda Galaxy begin to collide25 gal Sun ejects a planetary nebula, leaving behind a white dwarf30 gal The Milky Way and Andromeda complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda500 gal The Universe's expansion causes all galaxies beyond the Milky Way's Local Group to disappear beyond the cosmic light horizon, removing them from the observable universe2000 gal Local Group of 47 galaxies coalesces into a single large galaxy